GLaDOS is a robot from Portal and Portal 2. She is the main antanogist of Portal, forcing Chell to go through a series of pointless tests that ended with GLaDOS trying to kill her. In Portal 2, however, she was much nicer, instead helping destroy Wheatley during the second half of the game. At the end of Portal and Portal 2, she sang the songs Still Alive and Want You Gone (respectively), which was loved by fans of the series.

GLaDOS was based off of the personality of Caroline, the secretary and close assistant of Cave Johnson. She becomes much nicer under the influence of her personality, but ultimately deletes it at the end of the game.

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Remember when the platform was sliding into the fire pit and I said 'Goodbye' and you were like [no way] and then I was all 'we pretended we were going to murder you'? That was great.”

— GLaDOS • Portal

GLaDOS as she appears in Portal

GLaDOS is the main antagonist in Portal. She guides the player throughout a series of tests, appearing to be a series of pre-recorded messages. However, some of these messages have an odd note to it (messages shorting out at important times, not reacting when Chell is clearly in mortal danger), hinting that something strange and possibly sinister is going on.

GLaDOS motivates her test subject with the promise of Cake and grief counseling once the tests are over. However, any rewards she promised are, in fact, lies. Instead of the aforementioned rewards at the end of the testing tracks, she instead sends the subject on a moving platform headed towards an incinerator.

The protagonist of the game, Chell, manages to outsmart GLaDOS and escape, prompting her to be more emotionally expressive, showing alarm and claiming that she is "pleased that [Chell] made it through the final test where we pretended we were going to murder you." As Chell's escape progresses, GLaDOS finally shows her true colors and declares: "Turn back or I will kill you. I'm going kill you, and all the cake is gone."

When Chell finally shows up to confront GLaDOS in her lair, the latter tricks her into incinerating her Morality Core, freeing her from its restraints and allowing her to flood the Enrichment Center with a deadly neurotoxin. The Morality Core also had a few ancillary responsibilities, as GLaDOS was incapable of shutting off the Rocket Turret that had been activated upon the core's destruction. This is a key component for defeating her in battle, as Chell must redirect the rockets to knock Personality Cores off of GLaDOS and incinerate them. Once that has been acheived, they are both blown up to Aperture's parking lot, but before Chell can make her escape, she is dragged back into the facility by a Party Escort Robot.

After a quick tour of Aperture's interior, the player will see a room, with a Weighted Companion Cube and a cake. Then, multiple Personality Cores light up, and then a robotic arm drops down and extinguishes the candle on the cake, which was not a lie after all.

Why do I hate you so much? You ever wonder that? I'm brilliant. I’m not bragging. It's an objective fact. I'm the most massive collection of wisdom and raw computational power that’s ever existed. And I hate you. It can't be for no reason. You must deserve it.”

— GLaDOS • Portal 2

GLaDOS has a bit of a mixed relationship with Chell. She regareded her as a simple test subject before she escaped and "murdered" her. After this, GLaDOS harbored a deep hatred towards her until she realized her previous personality, Caroline.

After this, she became much nicer to Chell. After deleting Caroline, GLaDOS decides to set Chell free and even returns her original Companion Cube.

GLaDOS can be described as openly sadistic, often making jokes about or simply having no reaction at all to situations that could harm test subjects to a major degree. She enjoys using reverse psycology and cruel jokes to discourage Chell throughout Portal, although her attitude towards Chell softens somewhat during the events of Portal 2. Whether the experiments she conducts on individuals within the facility have any use or are just done for the sake of it are unknown, but she has a sincere passion for science, which she regards as her sole purpose in her A.I "life."